1. Field of the Invention
The present invention relates to a digital circuit for providing a binary signal on the occurrence of a predetermined frequency ratio, v=f1/f2, of two signals F1, F2 during a selectable measuring period and, more particularly, to such a circuit as used in a color-television receiver.
2. Description of the Prior Art
A digital circuit of the aforementioned type is needed in color-television receivers to determine the ratio between the chrominance-subcarrier frequency and the horizontal frequency, which ratio is fixed at the transmitting end both in the PAL system and in the NTSC system. This "recognition" signal can be used to lock the horizontal deflection oscillator, via a frequency divider dividing by a fixed number, to the chrominance-subcarrier oscillator, designed as a phase-locked loop, during a color-television broadcast, so that advantage can be taken of the considerably better noise immunity of the phase-locked loop, which includes a crystal oscillator.
In the case of a nonstandard ratio between chrominance-subcarrier frequency and horizontal frequency, however, it must be possible, like in conventional circuits, to synchronize the horizontal deflection oscillator by means of the horizontal synchronizing pulses separated from the transmitted composite color signal. This is particularly important because even under normal transmitting conditions switchover between the standard ratio and a nonstandard ratio of the two signals or horizontal-and field-frequency phase shifts may occur at the transmitting end. To avoid constant switching between the two above-described modes of synchronization of the horizontal deflection oscillator, the digital circuit must be able to reliably recognize normal television signals, in which the frequency ratio is only in the vicinity of the predetermined or standard frequency ratio, as signals with a nonstandard ratio, i.e., it must have a high degree of accuracy.
While the problem underlying the invention was just explained in connection with color-television receivers, the use of the invention is not limited to this specific case, which represents only a preferred application of the invention.